What is the Diameter symbol in GD&T?

⌀ — Unicode U+2300 (DIAMETER SIGN). It goes in the first compartment of the feature control frame, ahead of the tolerance value.

Does Diameter need a datum?

Never references a datum. The tolerance zone floats to best-fit the feature, so this control says nothing about where the feature sits relative to anything else.

Which GD&T modifiers are in the same group as Diameter?

Diameter is one of 8 modifier modifiers: Diameter, Maximum Material Condition (MMC), Least Material Condition (LMC), Regardless of Feature Size (RFS), Projected Tolerance Zone, Free State, Tangent Plane, Unequally Disposed Profile.

GD&T · Modifier · ASME Y14.5

Diameter GD&T Symbol — Meaning and Datum Rule

https://shopmath.org/gdt/diameter · data verified August 1, 2026

Diameter (⌀) is a GD&T modifier: it changes how a tolerance is interpreted rather than defining a zone of its own. It never references a datum.

Symbol: ⌀
Unicode: U+2300
Category: Modifier
Datum: never

Diameter · GD&T modifier symbol

⌀

Unicode: U+2300
Datum: never

Diameter qualifies another tolerance rather than standing on its own — it appears after the tolerance value or the datum reference it modifies.

What Diameter Controls in GD&T

Inside a feature control frame this makes the tolerance zone cylindrical rather than a pair of parallel planes. Its presence on a position callout is the difference between a round zone and a slab zone, and therefore between a hole pattern that gauges and one that does not.

The Diameter GD&T Symbol, Zone and Datum Rule

The symbol as it appears in the feature control frame, with the codepoint to type it and the rule it carries.

Diameter — GD&T symbol, tolerance zone and datum rule
Property	Value
Symbol	⌀
Unicode codepoint	U+2300 (DIAMETER SIGN)
Category	Modifiers and material conditions — Change how a geometric tolerance is interpreted or verified.
Tolerance zone	n/a — modifier
Datum reference	Never references a datum. The tolerance zone floats to best-fit the feature, so this control says nothing about where the feature sits relative to anything else.
Standard	ASME Y14.5 (cited, not reproduced); ISO 1101
Also called	diameter symbol gdt, phi symbol

The symbol character is the Unicode codepoint used in drawings and CAD annotations; drafting standards render it with defined proportions that the font approximates.

GD&T Modifiers Related to Diameter

Diameter is a modifier modifier . Change how a geometric tolerance is interpreted or verified. These are the others in that group — the ones a drawing usually chooses between.

GD&T modifier symbols related to Diameter
Symbol	Datum	Tolerance zone
Maximum Material Condition (MMC)	never	n/a — modifier
Least Material Condition (LMC)	never	n/a — modifier
Regardless of Feature Size (RFS)	never	n/a — modifier
Projected Tolerance Zone	never	Zone projected above the feature
Free State	never	n/a — modifier
Tangent Plane	never	n/a — modifier
Unequally Disposed Profile	never	Profile band split unevenly about the true profile

Ordered as they appear in the feature control frame reference, form through runout, then modifiers.

Sources and Method for the Diameter GD&T Reference

Diameter symbol and rulescompiled

Symbols and codepoints: Unicode Character Database (Unicode License v3). Characteristic list cross-checked against NASA MSFC GD&T Basics (public domain). Descriptions written from scratch; no ASME Y14.5 text reproduced.

Verified against Unicode Character Database codepoint check; 98/98 structural checks passed

Compiled from independent public sources that agree cell-for-cell.

Assumptions

The standard is cited, never reproduced: no ASME Y14.5 definitional text, worked example or figure appears on this site. The description above is original prose.
ASME Y14.5 and ISO 1101 differ in places — the ISO system uses the envelope requirement differently and does not carry every ASME modifier. Where a drawing names a standard, that standard governs.
Dataset scope: EXCLUDED: all ASME Y14.5 definitional text, worked examples and figures. Every description on this site is original prose.

Data last verified

August 1, 2026

How this data is built and checked →

How to Cite This Diameter GD&T Reference

Citation

ShopMath. "Diameter GD&T Symbol — Meaning and Datum Rule." ShopMath, verified August 1, 2026, https://shopmath.org/gdt/diameter

BibTeX

@misc{shopmath-gdt-diameter,
  title        = {Diameter GD&T Symbol — Meaning and Datum Rule},
  author       = {{ShopMath}},
  year         = {2026},
  howpublished = {\url{https://shopmath.org/gdt/diameter}},
  note         = {Data verified 2026-08-01}
}

Permanent URL

https://shopmath.org/gdt/diameter

Common Questions About Diameter in GD&T

What does Diameter mean in GD&T?: Inside a feature control frame this makes the tolerance zone cylindrical rather than a pair of parallel planes. Its presence on a position callout is the difference between a round zone and a slab zone, and therefore between a hole pattern that gauges and one that does not.
What is the Diameter symbol in GD&T?: ⌀ — Unicode U+2300 (DIAMETER SIGN). It goes in the first compartment of the feature control frame, ahead of the tolerance value.
Does Diameter need a datum?: Never references a datum. The tolerance zone floats to best-fit the feature, so this control says nothing about where the feature sits relative to anything else.
Which GD&T modifiers are in the same group as Diameter?: Diameter is one of 8 modifier modifiers: Diameter, Maximum Material Condition (MMC), Least Material Condition (LMC), Regardless of Feature Size (RFS), Projected Tolerance Zone, Free State, Tangent Plane, Unequally Disposed Profile.

Every GD&T symbol and modifier